I. Field of the Invention
This invention relates to code division multiple access (CDMA) communications, and more particularly to rate determination in a high capacity CDMA telecommunication system.
II. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or "mobile stations" and a fixed network infrastructure. One exemplary system is the well-known Code Division Multiple Access (CDMA) communication system. CDMA systems use unique code sequences to create communication channels in a spread-spectrum multiple-access digital communications system. The operation and functionality of CDMA systems is set forth in the Telecommunications Industry Association (TIA) specification governing CDMA operations, entitled "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System," TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, incorporated herein by reference, and referred to hereafter as "IS-95".
Communications from CDMA base stations to CDMA mobile stations use "forward CDMA channels" while communications from mobile stations to base stations use "reverse CDMA channels." The CDMA channels comprise access channels and traffic channels. These channels share the same CDMA frequency assignment using direct-sequence CDMA techniques. A distinct user channel long code sequence number identifies each traffic channel. The overall structure of a coherent reverse link CDMA traffic channel is shown in FIG. 1. A proposed forward link traffic channel that can be adapted for use with the present invention is similar to the reverse traffic channel of FIG. 1 and is described in more detail below. Data transmitted on the reverse CDMA channel is grouped into 20 ms frames. As shown in FIG. 1, prior to transmission, the reverse channel information bits are appended with cyclic redundancy codes (CRC) and "tail" bits. The information and tail bits are then encoded using a conventional encoding method to generate code symbols. Each code symbol is preferably a digital bit of information. In one exemplar of an encoder, four bits are output for each one bit that is input. Such encoders are typically referred to as 1/4 encoders. In one particular case, convolutional encoders are used to generate code symbols. The code symbols are repeated, block interleaved, and modulated prior to transmission. Each of the components in the reverse link traffic channel structure 100 is briefly described below.
In the exemplary CDMA traffic channel structure shown in FIG. 1, the data frames may be selectively transmitted on the reverse traffic channel at "basic" data rates of 9600 ("rate 1"), 4800 ("rate 1/2"), 2400 ("rate 1/4") and 1200 ("rate 1/8") bits-per-second. Higher data rates, such as 19.2 kbps ("rate 2"), 38.4 kbps ("rate 4") and 76.8 kbps ("rate 8"), may be supported by modifying the reverse traffic channel structure shown. An example of such an alternative reverse traffic channel is described below with reference to FIG. 7. The basic data rates are generated after frame quality indicators and encoder "tail bits" are added to the information bits by blocks 102 and 104, respectively. The frame quality indicators comprise cyclic redundancy codes (CRC) which support two functions: (1) assist in determination of whether a frame is transmitted in error, and (2) assist in the determination of the transmitted data rate in the receiver. The number of CRC bits added depends upon the basic data rate being used.
Other rate determination metrics are required to perform data rate determination in the receiver. In some systems, not all frames contain CRCs. For example, in the structure of FIG. 1, the two lowest data rates (1.2 and 2.4 kbps rates) do not include CRC information. In addition to the CRC information, symbol error rates (SER) evaluated at the four candidate basic data rates have been used for rate determination. In addition, prior art systems have used energy metrics to aid rate determination in the receiver. Disadvantageously, due to correlation in data transmitted at the various rates (especially for long zero strings), data rate determination has proven difficult using these rate determination metrics.
The encoder tail bits are simply eight logical zeros that are appended to the end of each frame. The tail bits are appended to frames by the encoder tail block 104. The data frames are input to an encoder block 106 as shown in FIG. 1. The reverse channel may use any of the candidate basic data rates to transmit data. The basic data frames comprise 24 bits (for 1.2 kpbs data rate), 48 bits (for 2.4 kbps), 96 bits (for 4.8 kbps) and 192 bits (for 9.6 kbps). The encoder 106 can be implemented using any convenient well known encoding technique. For example, a convolutional encoder can be used to implement the encoder 106 of FIG. 1. In this case, the convolutional code is preferably rate 1/4 and preferably has a constraint length of 9. The encoder 106 generates code symbols that are input to a basic rate repeater 108 as shown in FIG. 1.
The basic rate repeater 108 repeats the information which is encoded at the lower rates to ensure transmission at a fixed rate. Consequently, the over-the-air transmission rate is the same for every user regardless of the rate at which actual information is being transmitted. The basic rate repeater 108 repeats the code symbols before they are interleaved. In the reverse link traffic channel structure 100 shown in FIG. 1, each code symbol at the 9.6 kbps rate is repeated once (i.e., each symbol occurs two consecutive times). Each code symbol at the 4.8 kbps rate is repeated thrice (i.e., each symbol occurs four consecutive times). Each code symbol at the 2.4 kbps rate is repeated seven times (i.e., each symbol occurs eight consecutive times). Each code symbol at the 1.2 kbps rate is repeated fifteen times (i.e., each symbol occurs sixteen consecutive times). This results in a constant code symbol rate of 76,800 code symbols per second. The repeated code symbols generated by the basic rate repeater 108 are input to the block interleaver 110 prior to transmission.
The block interleaver 110 functions in a well-known manner to create a pseudo-random temporal separation between adjacent code symbols. The block interleaver 110 distributes the code symbols over a period of time to make the transmitted data more robust and thereby more resistant to bursty errors and adverse channel fading characteristics. This ensures that data can be accurately transceived under a variety of adverse channel conditions. The code symbols are modulated by the modulator 112 prior to transmission.
Disadvantageously, the reverse link traffic channel structure shown in FIG. 1 makes rate determination at the receiver very difficult to perform. Because symbols are simply repeated by the basic rate repeater 108, rather than encoded or covered, the de-interleaved codes are highly correlated among the different data rates, especially when the codes produce long strings of logical zeros or ones. The zero strings are highly correlated because the same zero strings are produced by the basic rate repeater 108 when using any of the candidate basic data rates. For example, a rate 1/4 all-zeros code repeated eight times will appear the same as a rate 1/2 all-zeros code repeated four times. The same zero string is generated by the basic rate repeater 108 in both cases. Disadvantageously, the two code symbol sequences will cause receivers to error when attempting to determine the rate at which the data is transmitted. The rate determination errors create problems at the receiver and thereby produce decoding errors. Therefore, an improved traffic channel structure including a rate covering technique is needed to facilitate rate determination in the receiver.
Further, the prior art data rate determination metrics have failed to produce reliable results especially when the data contains long strings of zeros or ones. Therefore, a technique is required which will improve the performance of the prior art data rate determination metrics.
The present invention provides such an improved rate determination method and apparatus.